staging: comedi: use refcount in sysfs attribute handlers

Call `comedi_dev_get_from_minor()` instead of `comedi_dev_from_minor()`
in the sysfs attribute handler functions to increment the reference of
the `struct comedi_device` during the operation.  Call
`comedi_dev_put()` to decrement the reference afterwards.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>